Recently, as existing energy sources such as oil and charcoal and so on are expected to be exhausted, attention is now paid to alternative energy sources which can be used in place of the existing energy sources. Among the alternative energy sources, sunlight energy is most abundant and has no environmental pollution. For this reason, more and more attention is paid to the sunlight energy.
A photovoltaic device or a solar cell converts sunlight energy directly into electrical energy. In performing this conversion, the photovoltaic device uses mainly the photovoltaic effect of a semiconductor junction. In other words, when light is incident and absorbed to a semiconductor p-n junction formed through a doping process by means of p-type and n-type impurities respectively, light energy generates electrons and holes at the inside of the semiconductor. Then, the electrons and the holes are separated by an internal field so that a photo-electro motive force is generated at both ends of the p-n junction. Here, if electrodes are formed at the both ends of junction and connected with wires, an electric current flows externally through the electrodes and the wires.
In order to substitute the existing energy sources such as oil with a sunlight energy source, it is required that a degradation rate of the photovoltaic device be low and a stability efficiency of the photovoltaic device be high. This low degradation rate and high stability efficiency are produced by the elapse of time.